1. Field of the Invention
The present invention relates to a playback apparatus, a recording medium and a producing method of the recording medium, which are applied to the technical field wherein the playback of content data is controlled on the basis of playback period information attached to that content data.
2. Description of the Related Art
A content data playback apparatus can be provided with not only a content data playback function but also a clock function which uses an internal clock to display a date and time. A content data playback apparatus with such a clock function is in wide use.
To control the period of time during which content data is playable, it is thought to provide this type of playback apparatus with a playback control function. The playback control function is a function of controlling a playback-permitted period for each content data by comparing playback-permitted period information which a content provider assigns to content data with a value of the internal clock.
However, the above playback apparatus may have problems if the user can freely change the value of the internal clock. In this case, the user can change the value of the internal clock to set a playback-permitted period each time he or she wants to play content data. Although changing that value is somewhat troublesome to the user, it is nonetheless possible to play the content data for ever.
To solve this problem, it is thought to provide the playback apparatus with a so-called “absolute clock”, which accurately measures time and does not allow the user to change the value. The absolute clock can accurately measure time over a period of one decade without reference to power outages which may occur in the meantime. However, the playback apparatus incorporating such an absolute clock is inevitably high in producing cost.
A “radio clock”, which resembles the absolute value in function but is lower in terms of cost, is also known in the art. The radio clock regularly receives a radio wave representing a reference time and adjusts the present time of the clock in accordance therewith. Hence, the time indicated by the radio clock can be regarded as substantially accurate.
However, the radio clock is not reliable where it cannot receive radio waves. Since the playback apparatus is often used in indoor areas radio waves do not reach, it is not appropriate to incorporate the radio clock in the playback apparatus.